<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    .Main{
      width: 100%;
      height: 700px;
      border: 1px green solid;
      text-align: center;
    }
    .show{
      width: 75%;
      height: 300px;
      border: 1px green dashed;
      margin: 20px auto;
    }
    .text{
      font-size: 30px;
      color: green;
    }
    .input{
      width: 70%;
      height: 150px;
    }
  </style>
</head>
<body>
<div class="Main">
  <div class="text">留言板</div>
  <div class="show"><div id="insert"></div></div>
  <div class="input" id="input"><textarea></textarea></div>
  <div class="button"><button id="btn">提交</button></div>
  <script>
    var btn=document.getElementById("btn");
    var insert =document.getElementById("insert");
    btn.onclick = function(){
      console.log("clicked");
      var txt=document.getElementById("input").value;
      var create=document.createElement('input');
      create.value=txt;
      insert.appendChild(create);
    }
  </script>
</div>
</body>
</html>